1. Field of the Invention
The present invention relates an image processing method and an image processing apparatus that is applicable to an image capturing apparatus and that reduce noise.
2. Description of the Related Art
In recent years, there have been strong demands for shooting techniques for still cameras and video cameras under lower light intensity and for shorter exposure time than before. However, when an image is shot with a low light intensity and for a short exposure time, since the amount of incident light is small, the charge storage amount is low. To obtain a bright image, it is necessary to amplify the captured signal. At this point, noise that occurs in an image sensor and an analog circuit is also amplified. As a result, a noisy low quality image is obtained. To solve this problem, an image is shot for a long exposure time such that the charge storage amount increases. Random components are relatively decreased such that S/N ratio is improved. However, when an image is shot for a long exposure time, the image blurs due to hand vibration during exposure (referred to as exposure blurring). Thus it was necessary to fix an image capturing apparatus with a tripod or the like.
As an alternative method, images may be serially shot at a blurring-free shutter speed, for example, of 1/30 seconds and then a plurality of images that have been serially shot are averaged to decrease noise that does not correlate with the time direction. However, there may be problems of deterioration such as image blurring due to moving of a camera and a moving object and multi-exposure afterimages due to a moving object.
In addition, Japanese Patent Application Laid-Open No. HEI 9-261526 (referred to as patent document 1) discloses a technique of correcting a plurality of hand-vibrated images serially shot and overlaying them. In this case, hand vibration occurs in an exposure time, for example in one field or one frame. Likewise, Japanese Patent Application Laid-Open No. HEI 11-75105 (referred to as patent document 2) discloses a technique of dividing the whole exposure time into a plurality of portions, correcting hand-vibrated images obtained in each portion of the whole exposure time, and adding the corrected images to obtain a high quality image.